heating

Physics Chemistry Biology Vocabulary Links Help

Physical and chemical change

Table to show the effect of heat on some chemicals

Chemical	Appearance	Effect of heat	Type of change	Change in mass when heated.
Magnesium ribbon	Grey metal	Burns with an intense white flame to leave a white ash called magnesium oxide.	Permanent	Gain because it combines with oxygen from the air
Copper foil	Pink metal	Forms a layer of black copper oxide on the surface.	Permanent	Gain because it combines with oxygen from he air.
Hydrated cobalt chloride	pink/purple crystals	Turned to a blue powder called anhydrous cobalt chloride and gave off water vapour. The blue powder gradually turned pink again when it was left standing in the room.	Reversible chemical change. Often used as a test for water *	Loss because it loses water vapour.
Potassium permanganate	dark purple/black crystals	Decomposes and produces oxygen gas	Permanent	Loss because it loses water vapour
Calcium carbonate (chalk, limestone)	White solid	Decomposes, producing carbon dioxide gas and leaving a white solid called calcium oxide (lime)	permanent†	Loss because it loses carbon dioxide
Hydrated copper sulphate	Blue crystals	Turned to a white powder called anhydrous copper sulphate and produced water vapour.	Reversible chemical change.	Loss because it loses water vapour.
Zinc oxide	White powder	Turned yellow when hot and white when cold.	Temporary	No change.
Iodine	Dark grey/purple crystals	Forms a purple vapour which turns back to grey iodine crystals when it cools.	Temporary	No change because all of the iodine vapour turns back to solid iodine crystals.
Copper carbonate	Green Powder	Decomposes. Evolves carbon dioxide gas and leaves a black residue of copper oxide.	Permanent	Loss because it gives out carbon dioxide gas.

*Test for the presence of water: Water can be identified by placing it onto blue cobalt chloride paper which will turn pink

†Calcium carbonate can be obtained from lime by dissolving the lime in water, filtering and blowing carbon dioxide through the filtrate.

Types of change that could occur when a single substance is heated:
The change could either be PHYSICAL or CHEMICAL.

TYPES OF CHEMICAL CHANGE
A chemical change always produces a new substance and is usually permanent.

1. Decomposition
This is when a compound splits apart into two (or more) chemicals.

eg: copper carbonate decomposes when heated to form carbon dioxide gas and leave black copper oxide.

Word equation: Copper carbonate ---------> copper oxide + carbon dioxide

2. Oxidation
: When a chemical combines (joins up with) oxygen to form an oxide.

eg copper foil will oxidise when heated strongly in air

Word equation: copper + oxygen -------> copper oxide + water.

3. Combustion (or burning) is a kind of oxidation where a flame is usually produced.

eg Magnesium will burn in air to form magnesium oxide:

Word equation: Magnesium + oxygen -------> magnesium oxide

Signs for a chemical change are:

1. A change in colour ( eg changes from pink to blue)

2. A change in temperature (eg gets warm or hot)

Types of physical change
A physical change forms no new chemical and is usually temporary.

Getting hotter
MELTING : When a solid turns to a liquid. This happen as soon as the substance reaches (or goes above) a temperature called its melting point
eg WAX melts when heated.

EVAPORATING: When a liquid turns to a vapour

BOILING: This is when a liquid turns to a vapour at its boiling point.
eg WATER boils when heated.

SUBLIMING : This is when a solid turns to a vapour without becoming a liquid
eg IODINE turns from a solid to a purple vapour when heated

Getting colder
CONDENSING: This is when a vapour turns to a liquid when it is cooled down (to below the boiling point)

FREEZING (or solidifying): This is when a liquid turns to a solid. This happens when the liquid cools to below its melting point.

SUBLIMING : This is when a vapour turns to a solid without becoming a liquid
eg IODINE turns from a purple vapour to a black solid when heated

[notice that the same word (subliming) is used for the change from a solid to a vapour and from a vapour to a solid]